Poly (alkylene carbonate) is an easily biodegradable polymer and is useful for packaging or coating materials. Methods of preparing poly(alkylene carbonate) from an epoxide compound and carbon dioxide are highly eco-friendly in that phosgene which is harmful compound is not used and carbon dioxide is obtained at a low cost.
Since 1960's, many researchers have developed various types of catalysts to prepare poly(alkylene carbonate) from an epoxide compound and carbon dioxide. Recently, a catalyst having high activity and high selectivity and synthesized from a Salen: ([H2 Salen=N,N′-bis(3,5-dialkylsalicylidene)-1,2-ethylenediamine]-type ligand containing a quaternary ammonium salt has been published [Korean Patent Registration No. 10-0853358 (Registration Date: Aug. 13, 2008); Korean Patent Application No. 10-2008-0015454 (Filing Date: Feb. 20, 2008); PCT/KR2008/002453 (Filing Date: Apr. 30, 2008); J. Am. Chem. Soc. 2007, 129, 8082-8083 (Jul. 4, 2007); Angew. Chem. Int. Ed., 2008, 47, 7306-7309 (Sep. 8, 2008)]. The catalyst disclosed in Korean Patent Registration No. 10-0853358 shows high activity and high selectivity, and may provide a copolymer having a large molecular weight and may be polymerized at a high temperature to be applicable to commercial processes. Furthermore, this catalyst is advantageous because a quaternary ammonium salt is contained in the ligand, and thus the catalyst may be easily separated from a copolymer resulting from copolymerization of carbon dioxide and epoxide, and re-used.
Also, the inventors of Korean Patent Registration No. 10-0853358 have carefully examined a structure of a particular catalyst having higher activity and higher selectivity as compared to other group among the catalyst group disclosed in the above patent, and have proved that such a catalyst has a peculiar structure in which nitrogen of the Salen ligand is not coordinated to a metal but oxygen thereof only is coordinated thereto, which was not known until now (see Structure 1 below, Inorg. Chem. 2009, 48, 10455 10465).

Furthermore, a method of easily synthesizing the ligand of the compound of Structure 1 has been developed (Bull. Korean Chem. Soc. 2009, 30, 745-748).
Poly(alkylene carbonate) having a high molecular weight may be economically prepared using the compound of Structure 1 as a highly active catalyst. However, since poly(alkylene carbonate) has a low glass transition temperature (which is 40° C. in the case of poly(alkylene carbonate) prepared from propylene oxide and carbon dioxide) and has insufficient mechanical strength, predetermined limitations are imposed on the applications that may be developed therewith.
With the way of overcoming the limitations of poly(alkylene carbonate), methods of preparing poly(alkylene carbonate)polyol having a low molecular weight and a plurality of —OH terminal groups and preparing polyurethane therefrom have been developed. Polyurethane is a polymer obtained by reacting a compound having an —OH group with a compound having an isocyanate (—NCO) group thus forming a urethane bond (—NHC(O)O—). A variety of compounds having an —NCO group have been used, a variety of compounds having an —OH group have been developed, and thermoplastic or thermosetting plastics or elastomeric polyurethanes having various physical properties have been developed and used. Examples of the compound having an —OH group mainly represent diols and polyester diols having —OH terminal groups at end groups, which have thousands of molecular weights obtained by ring-opening polymerization of ethylene oxide or propylene oxide. Attempts have been made to prepare poly(alkylene carbonate)diol or polyol to thereby attain polyurethane rather than poly(alkylene oxide)diol or polyester diol (W. Kuran, Polymeric Materials Encyclopedia, J. C. Salamone, Ed. CRC Press, Inc., Boca Raton 1996, Vol. 9, p. 6623; Polymer, 1992, vol 33, 1384-1390). Polyurethane prepared from poly(alkylene carbonate)polyol is known to have higher hydrolyzability compared to polyurethane prepared from polyester polyol (European Patent No. 302712; U.S. Pat. No. 5,863,627), and is also reported to have greater antistatic effects (U.S. Pat. No. 4,931,486). Furthermore, thrombus coagulation resistance is reported to be high (PCT International Patent Laid-Open Publication No. 9857671).
European Patent No. 302712 (priority date: Aug. 4, 1987) and European Patent No. 311278 (priority date: Oct. 6, 1987) disclose polycarbonate diol prepared by condensing diethylcarbonate (EtOC(O)OEt) and 1,6-hexanediol or 1,5-petandediol, and preparation of polyurethane using the same. In addition, U.S. Pat. No. 5,171,830 (filing date: Aug. 16, 1991) discloses a method of synthesizing poly(alkylene carbonate) by condensing dialkyl carbonate (ROC(O)OR) and alpha, omega-alkanediol having 4 or more carbons in the presence of a base catalyst and preparation of a urethane resin using the same.
European Patent No. 798328A2 (priority date: Mar. 28, 1996) discloses synthesis of polycarbonate-co-polyether diol using condensation of polyether diol and dimethylcarboante (MeOC(O)OMe).
Also, synthesis of poly(alkylene carbonate)macrodiol by condensation of various diol compounds and ethylene carbonate and preparation of polyurethane using the same are disclosed in J. Appl. Polym. Sci. 1998, 69, 1621-1633 and J. Appl. Polym. Sci. 1989, 37, 1491-1511.
However, such poly(alkylene carbonate)polyol is not prepared using copolymerization of carbon dioxide and epoxide and also has a different structure from that of a copolymer of carbon dioxide and epoxide. Specifically, in order to prepare poly(alkylene carbonate) using condensation of ethylene carbonate or dialkyl carbonate, diol having spaced 3 or more carbons should be used. That is, poly(alkylene carbonate) has a structure in which a carbonate bond is linked by 3 or more carbons. Poly(alkylene carbonate) prepared by copolymerization of carbon dioxide and epoxide has a structure in which a carbonate bond is linked by 2 carbons.
U.S. Pat. No. 4,686,276 (filing date: Dec. 30, 1985) discloses a method of synthesizing poly(ethylene carbonate)diol by copolymerizing carbon dioxide and ethylene oxide in the presence or absence of ethylene carbonate using a diol compound as an initiator and a catalyst consisting of an alkaline compound and a tin compound. In addition, U.S. Pat. No. 4,528,364 (filing date: Apr. 19, 1984) discloses a method of removing a catalyst from the prepared polymer compound. Here, the prepared polymer has a carbon dioxide content less than 30%, which is not a complete alternating copolymer. In addition, preparation of polyurethane using poly(ethylene carbonate)diol which was prepared and purified by the above method is disclosed in Journal of Applied Polymer Science. 1990, 41, 487 507.
European Patent No. 0222453 (filing date: Jun. 11, 1986) discloses a method of synthesizing polyol by copolymerizing carbon dioxide and epoxide using a double metal cyanide compound as a catalyst and using an organic material having reactive hydrogen as a molecular weight regulator. However, the obtained polyol has a carbon dioxide content of 5 to 13 mol %, which is not a pure poly(alkylene carbonate) compound based on complete alternating copolymerization of carbon dioxide and epoxide.
CN Patent No. 1060299A (filing date: Sep. 19, 1991), which was published later, discloses a preparation method of polyol by copolymerizing carbon dioxide and epoxide using a polymer-supported bimetallic catalyst and using an organic material having 1 to 10 reactive hydrogen as a molecular weight regulator. However, the polyol prepared by Examples has a carbon dioxide content of 37 to 40 mol %, which is not a pure poly(alkylene carbonate) compound based on complete alternating copolymerization of carbon dioxide and epoxide.
U.S. Pat. No. 8,247,520 (filing date: Sep. 8, 2009) discloses a method of copolymerizing carbon dioxide and epoxide using a chain transfer agent which is a molecular weight regulator under a binary catalyst system of (Salen)Co compound. However, the present inventors found that as an amount of the used chain transfer agent becomes increased in the copolymerization system, catalyst system activity is deteriorated, such that there is a limitation in obtaining a low molecular weight of copolymer having desirable level.
As described above, synthesis of a low molecular weight of poly(alkylene carbonate) by copolymerization of carbon dioxide and epoxide in the presence of a molecular weight regulator has been abundantly reported. Meanwhile, in order to prepare appropriate poly(alkylene carbonate) having a molecular weight required in a large-scale commercial process, since maintenance of catalyst system activity in the preparation process as well as economical cost of copolymerization catalyst system are important, development of a novel catalyst system capable of satisfying the requirements has been demanded.